The present invention generally relates to cooling chip packages, and more particularly to cooling chip stacks by employing an onboard cooling structure.
In general, it is important to cool semiconductor chips, such as processor chips, to maintain reliable operation, reduce leakage currents, and prevent thermal damage to electronic components. It is more problematic and difficult to implement effective mechanisms for cooling 3D chip stacks as compared to singular chips, and the ability to efficiently cool a chip stack can limit the height and total power of a chip stack. Common cooling techniques for chip stacks include the use of high-performance water cooling systems on a backside of the chip stack, but this technique may not be adequate for a stack structure with many chips or a chip stack having a high-power chip on the bottom of the stack. While a water-cooled thermal interposer can be used at the bottom of the chip stack, this structure is difficult to integrate and may require isolation of thru silicon vias (TSVs) from the liquid coolant that is used. If a dielectric fluid is used as the coolant, isolation of the TSVs is not required. However, with single phase cooling, the performance of dielectric fluids is inferior to water.
Other cooling techniques include two-phase cooling in which a liquid coolant having a relatively low boiling point is used (e.g., liquid which evaporates at an operating temperature of the chips being used). With two-phase cooling in closed channels, the heated liquid evaporates to create an annular flow whereby a thin liquid film (evaporation layer) is present on the surfaces being cooled, and heated evaporated coolant flows through confined channels to outlet ports. As such, compared to pure liquid (e.g., single phase) cooling techniques, two-phase cooling can provide greater cooling ability using a much lower volume of coolant fluid, lower coolant mass flow rates and lower operating pressure drop. Advantages of two-phase cooling include the ability to select the boiling temperature of the coolant or use an expansion valve for refrigeration.
Another cooling technique includes expanding radial channels to mitigate the acceleration in the stream-wise direction of the fluid cavity. Accordingly, the pressure drop and the dry-out risk can be reduced in the heat transfer cavity. However, a central hole is required in a chip stack to feed the refrigerant into the radial expanding channels.